1. Field of the Invention
Embodiments of the present invention generally relate to three-dimensional solid-state memory and a method for addressing memory cells in a three-dimensional arrangement.
2. Description of the Related Art
Phase change memory (PCM) is a type of non-volatile memory technology. PCM is an emerging technology and a candidate for storage class memory (SCM) applications and a serious contender to dislodge NOR and NAND flash memory in solid state storage applications and, in the case of NAND flash, solid-state drives (SSDs). PCM functions based upon switching a memory cell, typically based on chalcogenides such as Ge2Sb2Te5, between two stable states, a crystalline state and an amorphous state, by heating the memory cell. To heat the memory cell, an electrical current flows through the PCM cell. For an effective memory device, numerous PCM cells will be present in an array. Each of the PCM cells needs to be addressed, programmed and read with low overhead electrical wiring. The PCM cell is the phase-change cell itself, and PCM device, as discussed herein, is the set of PCM cells plus accompanying heaters (represented by a resistor in the electrical diagrams). The PCM device is the memory element herein.
An array 100 of PCM cells is frequently arranged with a selecting transistor 102 in series with each memory cell 104 as shown in FIG. 1A. Word lines (WL) and bitlines (BL) are arranged so that each memory cell 104 can be programmed or queried. A row of PCM cells is activated by a single word line WL and each one of the PCM cells 104 in that row will affect the bitline BL to which it is electrically connected according to the state of the PCM cells 104, i.e. according to the PCM cells 104 being in their high (amorphous) or low (crystalline) resistance state. As shown in FIG. 1A, a simple array 100 of PCM devices 106 is shown. The array 100 is a two dimensional array because the PCM devices 106 are all arranged along a common plane.
In an alternative design commonly named “cross-point”, shown in FIG. 1B. Each interception of word lines WL in the x direction and bit lines BL in the y direction has a PCM device 106, which includes the PCM cell 104 itself and its heater (represented by a resistor). Frequently, a selecting device is added in series with the PCM device. This selecting device can be a diode or a transistor. The selecting device, diode or transistor, added to the cross-point array 110, or alternatively, used externally to the array of PCM cells may frequently become the limiting factor on how dense can the PCM array become.
When the selecting device is added to the cross-point array, there will be one selecting device per PCM device 106. Current requirements of the PCM device 106 need to be met by the selecting device. In consequence, even when the PCM device 106 can be made small to the lithographic limit and occupy only 4F2 of area, where F is the half-pitch critical dimension in a lithographic technology, the selecting device might require 30F2 if it is a CMOS transistor or 10F2 if it is a bipolar transistor. Optimized diodes, where efforts to make them very conductive might attend the current requirement of a PCM device using 4F2 area and are therefore very frequently considered as selecting device in cross point memories using PCM or any memory device requiring significant currents for operation.
Unfortunately, using diodes makes it very difficult to extend the concept of cross-point array 110 from a two-dimensional (2D) array to a three-dimensional (3D) array. In a 3D array, addressing the PCM devices 106 that are in the middle of the array is difficult.
Therefore, there is a need for a PCM device that permits each PCM cell to be accessed individually while minimizing the use of the surface area of the substrate over which the PCM device is disposed as well as minimizing the overhead wiring utilized to address PCM cells in the middle of the PCM 3D array.